Ceramic tube

ABSTRACT

A long alumina ceramic tube having good linearity is made by longitudinally joining shorter alumina tubes at the ends thereof. The ends have tapered surfaces thereat to increase the contact surface of the joint. The contact surface has a thin coating of an alumina containing composition thereon which is fired at a high temperature to provide a secure joint between the shorter alumina tubes.

This invention concerns the manufacture of a ceramic tube for hightemperature use. In some applications, it is required that the tube bequite lengthy and quite true in linearity. In one such application,where a copper vapor atmosphere is maintained within the tube at 1500°C., a particular tube is 9' long by 3" inside diameter. In the past, thetube was made by slip casting in one piece. Because of the 9' length, itwas difficult to maintain the desired degree of linearity, especiallywhere the slip cast ceramic composition had to be sintered at a hightemperature. This invention discloses a method of making such a tubehaving improved linearity.

In this invention, the desired tube is made by first making shortertubes having predetermined dimensions and then joining the ends of theshorter tubes under conditions that will provide the improved linearity.The reason for the use of the shorter tubes is that it is easier tomaintain linearity in a shorter tube than it is in a longer tube. Theends of the tubes are ground to a taper that mates with each other. Thetapered ends of the tubes are then fitted together, with a thin layer ofa ceramic composition therebetween, and then sintered at a hightemperature to obtain a strong joint. Ideally, the ceramic compositionincludes the ceramic material of the tubes, for example, alumina, andthe sintering temperature is sufficiently high to obtain diffusion atthe joint and to approach obtaining a monolithic structure at the joint.

In one example, alumina tubes, about three feet long, were made byisostatic pressing A1998C (99.8% Al₂ O₃) alumina powder on a mandrel,removing the pressed tube, and then fully sintering at 1700° C. Thediameter of the mandrel was carefully selected so that after sintering,which resulted in about 20% shrinkage, the sintered tube had the desiredinside diameter of 3" and had, after outside machining, a wall thicknessof 1/4". The tube was then cut to a length of 37.4".

A taper was then ground into the end of the tube as shown in FIG. 1. Ontube 1, shown on the left side of FIG. 1, an inside taper 2 was groundon the right end of tube 1. In one example, taper 2 was 10°, and wallthickness 3 was 1/4". The 10° taper increased the length of the contactsurface area at tube ends from 1/4" to about 1.4", thereby considerablystrengthening the joint. A flat surface 4, about 10 mils long, wasground at each end of the taper to serve as a stop when tubes 1 and 5are butted for joining. Tube 5 is shown on the right side of FIG. 1. Anoutside 10° taper 6 was ground on the left end of tube 5. There werealso similar 10 mil long flat surfaces 7 ground at each end of taper 6.

The tapered surfaces of tubes 1 and 2 were coated with a joiningcompound consisting of finely ground calcium aluminate, alumina, anorganic vehicle and organic dye, and the coated ends were buttedtogether. Since the joining compound is only thinly applied, the dyeprovided color to aid in applying uniform coating. The excess coatingwas wiped off the inner and outer seams. The joint is then fired at ahigh temperature to bond the tubes together, as shown in FIG. 2.

Tubes 1 and 5, butted together, are placed inside clamshell furnace 8,the butted ends being about at the center of furnace 8 and the otherends of each tube protruding outside furnace 8. There are adjustablealumina supports 9 inside furnace 8 and adjustable supports 10 outsidethe furnace to support the entire lengths of tubes 1 and 5. Prior tofiring the furnace, supports 9 and 10 are adjusted to align tubes 1 and5 for accurate linearity. There is a stop 10 at the left end of tube 1and a spring loaded or hydraulic force 11 at the right end of tube 5 inorder to place the tubes in longitudinal compression during firing. Thisalso aids in keeping the thickness of the joint between the tapered endsquite thin. Ideally, the thickness of the joint after sintering shouldbe in the order of 1 or 2 mils. Alumina plugs 12, slightly smaller thanthe I.D. of the tubes, are inserted inside tubes 1 and 5 to about 12"from the joint. Their purpose is to reduce radiation heat loss. Thefurnace, burning natural gas and air, is then ignited and the joint isfired at about 1700° C. for about three hours under a longitudinal forceof about 150 to 300 grams per square inch. The calcium aluminate has aliquid phase at about 1500° C., and the liquid phase reacts with thealumina of the alumina tube to form a higher alumina composition, whichis non-liquid and more refractory than the starting calcium aluminate.

A third three foot long tube can be joined to the right end of tube 5 ina similar manner to provide the desired 9' tube having improvedlinearity.

In a second example, a 5° taper was used, instead of 10°. This increasedthe length of the contact surface area at the tube ends from 1.4" to2.8", thereby decreasing the load per unit contact area at the joint.

Tubes made of AL-995 alumina (99.5% alumina content) were evaluated withtwo different coating pastes, one made of 99.5% finely ground aluminaand the other made with the following mix.

Alumina, less than 1 um: 98.00 gm

Bentonite: 1.00

MgCO₃ : 1.00

Water (dry weight): 24.0

Ammonium Citrate (dry weight): 0.1

Few drops of red dye

The pastes were wet milled using 99.8% alumina media in 99.8 aluminamill for 24 hours. The pastes were then applied on the tapered sectionsand the tubes were assembled while wet. Using the equipment described inFIG. 2, the assembled tubes were fired at 1700° C. for 2 hours underconstant end loading, then slowly cooled. Both coating pastes gavesatisfactory results.

Six different pastes were evaluated for use with Al-995 tubes byapplying the pastes between AL-995 plates, firing at 1600° C. and thendye checking the joints between the plates for seam porosity. Bestresults were obtained with the 99.5% alumina paste and the 98% alumina,1% bentonite, 1% Mg CO₃ paste. The other four pastes, namely, AL-300(97.6% alumina), AL-600 (96% alumina), AL-500 (94% alumina) and calciumaluminate containing up to 30% of A-14 alumina (99.6% alumina content),also gave satisfactory results, but not as good.

I claim:
 1. The method of making an elongated alumina tube having apredetermined length comprising the steps of preparing two sinteredalumina tubes each having a length shorter than said predeterminedlength, grinding an outside tapered surface on one end of one of saidtwo sintered alumina tubes, grinding a mating inside tapered surface onone end of the other of said two sintered alumina tubes, thinly coatingsaid tapered surfaces with a composition including calcium aluminate andfinely powdered alumina, butting the ends of the two sintered aluminatubes together so that the tapered surfaces mate, firing said mated endsunder a compressive longitudinal force at a high temperature to promotealumina formation at the joint between the tapered surfaces and to forma secure joint therebetween.
 2. The method of claim 1 wherein the buttedsintered alumina tubes are aligned for linearity within a furnace priorto firing.
 3. The method of claim 1 wherein the ends of the taperedsurface have short flat surfaces thereat to provide stops when the twosintered alumina tubes are butted together.